In medicine and in the pharmacy it is often necessary to perform experiments with cell cultures. This applies, for example, to their culture, to their observation, their reaction on foreign and/or poisonous substances, to preservation and similar.
Furthermore the search for suitable organ replacements is becoming increasingly important.
One of the main areas is experiments regarding metabolic functions, especially of the liver.
The complexity of the large number of hepatocellular metabolic functions however places high demands on an artificial organ replacement for the liver. In the case of the artificial kidney, filtration and metabolic functions, which can be performed by equipment in the manner of dialysis, are of prime importance. Similarly with an artificial heart the pumping function in particular is replaced by a machine. The liver on the other hand has a large number of individual functions, which can be roughly divided into categories such as detoxifying function, protein secretion, endocrine functions, storage function, phagocytosis, fat and carbohydrate metabolism functions.
In known culture systems the liver cells, i.e. the hepatocytes, loose their functional ability within the first few days after isolation. Thus after just 2 to 3 days, depending on the function investigated, only roughly 80% and after 1 week only minimal residual functions remain. Later on cell death and proliferation with fibroblast-type cells occur. Previous experiments with liver cell cultures had to be performed in a phase with progressive cell degeneration.
To obtain the hepatocyte function in culture it has, for example, already been proposed practising epithelial co-culture, the addition of dimethylsulphoxide (DMSO) to the medium, or the use of a complex matrix (matrigel). However if the objective is the use of a liver cell culture which is to imitate as closely as possible the "in vivo" situation, a series of problems is produced by these conventional culture methods. Thus DMSO is a chemical substance also having a hepatotoxic action. Epithelial co-cultures are transformed cell lines and have an oncogenic character. It is therefore not possible to draw conclusions on the behaviour of naturally completely differentiated cells. Matrigel is in turn derived from sarcoma cell lines (Engelbrecht Holm sarcoma) and is not characterised in its components. A clinical use of oncogenic cells or their products (not defined in further detail) is therefore not worth striving for.
A system has already been proposed which generally consists of a hepatocyte monolayer with adhesion on one side to glass, plastic, or extracellular matrix containing protein as the cell culture slide.
A so-called sandwich culture system having a matrix-hepatocyte-matrix structure is also known. However for performance this system requires a surface as a support which has to be accessible for the application of the second upper layer. Devices on a base of hollow fibres or microcarriers in fact theoretically enable a mass culture, but are always used with conventional culture configurations. This causes a fast functional loss of the hepatocytes, and in addition significant oxygenation problems occur. A sandwich culture system can not be achieved in this manner, as adhesions would ensue.
A further disadvantage of the known cultures, especially of sandwich cultures, lies in that the oxygen supply to the cells could not adequately be ensured. In some regions there was under-supply, whereas in other regions there occurred an increase in the perfusion speed of the culture medium resulting in unwanted increases in transverse forces.
In order to avoid such oxygenation problems of cell structures, especially of hepatocytes in culture, gas-permeable membranes have already been proposed. In this case the cells, which lie on one side of the membrane, are supplied either by transmembrane air contact or via oxygen-enriched medium, which flows past the opposite side. However such individual membranes are only suitable for laboratory purposes or only for small quantities and sizes respectively.
A further main disadvantage of the known methods and structures lies in spatial problems, i.e. their large space requirement. Thus the removal of oxygenation and nutrient supply problems, for example, requires expensive separate pump circuits, which results in an enormous increase in size of the unit in relation to the number of cells actually cultivated. A mass culture is not possible in this manner, as a total organ replacement, e.g. of a human liver, with such a technology would have the spatial requirement of an entire house.
From EP 0 363 262 a device is known for the treatment of cell cultures using a three-chamber system inside a basic unit having a fixed housing. Two individual membranes separated from one another extend in this housing, as a result of which three chambers are produced. These membranes are separately attached in the walls of the basic unit. The cell culture space is in the central chamber, while the two other chambers represent supply chambers for the cell culture chambers.
However, a disadvantage is that, even with this device, the spatial requirement is considerable. Furthermore with this device it is not possible to achieve at least roughly an "in vivo" state for the cells.